The present invention relates to an improvement in a reactor tube for thermally cracking or reforming hydrocarbons.
When a liquid or gaseous hydrocarbon is subjected to the chemical reaction of thermal cracking or reforming at high temperature and high pressure in the presence or absence of a catalyst, solid carbon deposits, and the carbon deposit builds up in the lamellar form in the reaction zone on a surface of a tube serving as the reactor. While the following description will be given of the case in which solid carbon deposits on the inner surface of the tube, solid carbon will deposit on the outer surface or on both the inner and outer surfaces of the tube when the reactor is designed to effect the chemical reaction of hydrocarbons outside or inside and outside of the tube.
When a hydrocarbon is passed through the tube for the chemical reaction, the deposit of solid carbon on the inner surface of the tube, if allowed to build up, impedes the flow of a fluid containing the hydrocarbon. The carbon deposit also results in a seriously reduced heat transfer efficiency when reaction heat is supplied to or removed from the tube from outside to carry out the cracking or reforming chemical reaction. Consequently it becomes difficult to continue the operation of the reactor.
The material heretofore used for such reactors is Fe-Cr-Ni austenitic heat resisting steel which contains large amounts of Ni and Cr for use at high-temperature, high-pressure operating conditions and which is generally used for fabricating high-temperature equipment. It is usual practice to increase the Ni content of the heat resisting steel material for tubes which are used at higher operating temperatures.
However, when a tube is used which is produced from the Fe-Cr-Ni austenitic heat resisting steel, solid carbon inevitably deposits on the inner surface of the tube, giving rise to the necessity for shutdowns to decoke the reactor by various methods although the reactor is to be operated continuously as a rule.
The deposition of solid carbon occurs more markedly if the steel material has a higher Ni content. In this case, solid carbon deposits on the tube inner surface in a short period of time, and the carbon deposit builds up rapidly, consequently entailing the problem that the reactor must be decoked more frequently.
We have carried out intensive research on the deposition of solid carbon on the tube inner surface and found that there is a correlation between the amount of the solid carbon deposit and the Ni content of the Fe-Cr-Ni austenitic heat resisting steel material which forms the reactor tube and further that the Ni in the steel material, especially the Ni portion present on the inner surface of the tube, acts catalytically to promote the deposition of solid carbon from hydrocarbons.